<html>
  <head>
    <script>
      // Called when the url of a tab changes.
      function checkForValidUrl(tabId, changeInfo, tab) 
      {
          // If 'amazon' is found in the tab's URL...
          if ( tab.url.indexOf('amazon') > -1 ) 
          {
              // ... show the page action.
              chrome.pageAction.show(tabId);
          }
      };

      // Listen for any changes to the URL of any tab.
      chrome.tabs.onUpdated.addListener(checkForValidUrl);
    </script>
  </head>
</html>
